Adventus Group

In Situ Chemical Reduction (ISCR) Technologies: Significance of Low Eh Reactions

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Courtesy of Adventus Group

As used herein, ISCR describes the synergistic effect of stimulated biological oxygen consumption (via degradation of added organic carbon sources), direct chemical reduction with reduced metals, and the corresponding enhanced thermodynamic decomposition reactions that are realized at the lowered redox (Eh) conditions. These combined effects are therefore characteristic for technologies that combine controlled-release carbon plus ZVI or another reduced metal, e.g. Zn(0) (Seech et al., 1995 and 2000) or other reducing agents (Szecsody et al., 2004) such as dithionite (Nzegung et al., 2001). Following placement of ISCR reagents into the subsurface environment, a number of physical, chemical and microbiological processes combine to create very strong (e.g., Eh < −,550 mV) reducing Address correspondence to Jan Dolfing, School of Civil Engineering and Geosciences, Newcastle University, Newcastle NE1 7RU, UK.

1. biological reduction/consumption of oxygen and other electron acceptors like, e.g., nitrate and sulfate (via the biological oxygen demand generated by the addition of complex organic carbon),
2. (in)direct chemical reduction via reduced metals either via (direct) chemical reduction of the oxidized pollutants or (indirectly) via the formation of hydrogen, which is used by bacteria as the electron donor (Scherer et al., 2000), and
3. direct chemical oxidation via beta-elimination reactions and additional oxygen scavenging and reduced Eh via ZVI oxidation/reduction reactions.

Mineralization of halogenated organic compounds occurs via a combination of chemically induced hydrogenolysis, beta-elimination, and biological sequential reductive dehalogenation mreactions. As such, the accumulation of conventional catabolic intermediates is not observed, and ISCR reactions are more effective toward compounds resistant to individual processes. It is also possible, however, that unique, yet to be identified genes are expressed by dehalogenating microorganisms under these extreme physical conditions, and that such genes produce enzymes that catalyze these reactions (analogous to the synthesis of unique biomolecules under different physical conditions such as pressure and temperature). Studies aimed at evaluating this hypothesis seem warranted.

In any case, degradation routes are kinetically determined, and the type of catalyst and the presence of reactive ZVI surfaces will play important roles in these removal processes. The fermentation of the organic component present in the ISCR reagents liberates various organic acids and reduces naturally occurring reactive metals. This counters the production of hydroxyl ions resulting from the ZVI corrosion process, leading to more reactive ZVI surfaces. The importance of this “buffering capacity” will be discussed here and compared to various other induced anaerobic approaches that employ various carbon sources without the added benefit of ZVI.

It is also noteworthy that under ISCR conditions, the valence state of various inorganic compounds (e.g., heavy metals such as chromium(VI)) get reduced, which allows these materials to participate in various sorption and precipitation reactions. In the presence of ZVI and other elements such as sulfur, metals such as arsenic, lead and mercury can also be effectively immobilized in situ under ISCR conditions. The primary mechanism of removal is hypothesized to consist of the precipitation and co-precipitation of with iron/sulfur compounds. For example, arsenic is associated with the reduction of sulfate to form stable arsenopyrite (Craw et al., 2003, US EPA, 2000) thereby transferring it from the aqueous phase to the solid phase.

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